Survey of California Geopressured-Geothermal Potential

Description

Geopressured reservoirs contain three types of energy: thermal, hydraulic, and methane gas. The thermal energy generally is a function of depth of burial. It can be converted to electricity using the binary or flash power plant cycle, the flash technology being commercial only if the fluid temperature exceeds about 340 F. The hydraulic energy can be converted to electrical power using a hydraulic turbine. The dissolved gas can be separated and either used to produce electricity using a gas turbine or sold commercially. These reservoirs occur in many states in the USA, including California. An overburden pressure is caused by ...
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Description

Geopressured reservoirs contain three types of energy: thermal, hydraulic, and methane gas. The thermal energy generally is a function of depth of burial. It can be converted to electricity using the binary or flash power plant cycle, the flash technology being commercial only if the fluid temperature exceeds about 340 F. The hydraulic energy can be converted to electrical power using a hydraulic turbine. The dissolved gas can be separated and either used to produce electricity using a gas turbine or sold commercially. These reservoirs occur in many states in the USA, including California. An overburden pressure is caused by the combined weight of the formation rock and the fluids (water/gas/oil) present in the pore spaces overlying the formation of interest. The overburden pressure, in general, increases relatively uniformly with depth, whereas the hydrostatic gradient is mainly a function of two variables: the dissolved solids concentration and the temperature gradient. The hydrostatic pressure gradient for fresh water is 0.433 psi/ft. Geopressured reservoirs are overpressured; that is, the fluid pressure in the reservoir exceeds the pressure corresponding to the local hydrostatic pressure gradient. (Fig. 3) Confining bed or cap rock is necessary in order for a formation to be geopressured. Otherwise, the pressure would equalize to hydrostatic through upward flow. The pressures in a geopressured reservoir may approach the overburden pressure of about 1 psi/ft. Gulf Coast geopressured reservoirs typically exist between 12,000 to 20,000 feet below the surface. Flow rates of between 10,000 to 40,000 barrels per day, temperatures from 270 to 500 F, bottom hole pressures from 12,000 to 18,500 pounds psi, salinities of 20,000 to 200,000 milligrams per liter, and gas contents of 23 to 100 standard cubic feet per barrel, have been reported from geopressured wells.

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Proceedings, Geothermal Energy and the Utility Market - The Opportunities and Challenges for Expanding Geothermal Energy in a Competitive Supply Market; San Francisco, CA, March 24-26, 1992, Geothermal Program Review X

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